Improve CF Conformance: 2.7 Groups

[jamesdoyle21]

2.7 Groups
Requirements:

• The Conventions and external_variables attributes must not be used in non-root groups.
  Note: not in the test_cf.py it is under cf_1_8.py

@benjwadams note: this is due to the requirement being added in CF 1.8.

• If any dimension of an out-of-group variable has the same name as a dimension of the referring variable, the two must be the same dimension (i.e. they must have the same netCDF dimension ID).

• Variable or dimension paths must follow a UNIX style file convention. They must be formed of words (composed of letters, digits and underscores) and be separated by the slash character ('/'). Paths may begin with either '/', '…​' or a word.

• The variable or dimension referenced must exist in the file unless it is an external variable. References can be absolute, relative or with no path, in which case, the variable or dimension must be found in one of the following (in order of precedence):

In the referring group

In the ancestor group (starting from the direct ancestor and proceeding toward the root group, until it is found)

By the lateral search algorithm for coordinate variables only.

Recommendations:

• NUG-coordinate variables that are not in the referring group or one of its direct ancestors should be referenced by absolute or relative paths rather than relying on the lateral search algorithm.